The present invention relates to a glow plug, particularly of the kind for use in a diesel engine.
A glow plug for a diesel engine has an electric resistor or electric resistors, e.g., of coil type for performing a heating function and a temperature control function.
Research has been made on resistive material for such an electric resistor or electric resistors, which contains iron-group metals (i.e., iron, nickel and cobalt) as major components. This kind of resistive material has a positive temperature characteristic for electrical resistance so that the ratio of the electrical resistance at room temperature to the electric resistance at elevated temperature (hereinafter referred to as the resistance ratio) rises up with an increase in the temperature of the resistive material.
An example of such resistive material is disclosed in Japanese Laid-Open Patent Publication No. 58-83124 (JP-A-58-83124). According to JP-A-58-83124, a preheating plug has a heating resistance unit made of a Fexe2x80x94Nixe2x80x94Co alloy that comprises 40 to 70% by weight cobalt, 2 to 15% by weight nickel and the remainder of iron. As shown in FIG. 1 of JP-A-58-83124, the Fexe2x80x94Nixe2x80x94Co alloy has a resistance ratio that rapidly rises with temperature increase up to about 900xc2x0 C., and attains a good quick heating ability with toughness.
Another example of the resistive material is disclosed in U.S. Pat. No. 5,093,555. In accordance with U.S. Pat. No. 5,093,555, an electrical resistance element is made of a Fexe2x80x94Nixe2x80x94Co alloy essentially consisting of 20 to 35% by weight iron and the remainder of nickel and cobalt. In FIG. 1 of U.S. Pat. No. 5,093,555, the resistance ratios of iron and nickel are indicated by curves 4 and 5, respectively. As nickel has a lower Curie temperature than iron, the resistance ratio of nickel starts to settle down at a lower temperature around 400xc2x0 C. than that of Iron. Further, the resistance ratio of another alloy (i.e., a Fexe2x80x94Co alloy) is shown in FIG. 1A of U.S. Pat. No. 5,093,555. This Fexe2x80x94Co alloy contains 25% by weight iron, and has a resistance ratio that starts to increase rapidly at around 800xc2x0 C. and starts to settle down at around 900xc2x0 C. On the other hand, a family of Fexe2x80x94Nixe2x80x94Co alloys defined by U.S. Pat. No. 5,093,555 has resistance ratios as indicated by curves 1, 2 of FIG. 1 and curve 3 of FIG. 3. Each of the Fexe2x80x94Nixe2x80x94Co alloys exhibits a hysteresis curve in its resistance ratio-temperature characteristic so that the resistance ratio rises up to about 1000xc2x0 C. and starts to settle down at around 1000xc2x0 C. In other words, the Fexe2x80x94Nixe2x80x94Co alloy defined by U.S. Pat. No. 5,093,555 can attain a good quick heating ability up to about 1000xc2x0 C. and can perform a fine temperature control function from about 1000xc2x0 C.
Still another example is disclosed in U.S. Pat. No. 5,319,180. In accordance with U.S. Pat. No. 5,319,180, a sheathed-element glow plug has a resistor element composed of two resistor spirals. One of the resistor spirals performs a temperature control function, and is made of a Fexe2x80x94Co alloy containing 6 to 18% by weight iron (preferably, 12 to 14 % by weight iron), 81 to 94% by weight cobalt and any remainder not exceeding 1% by weight. This Fexe2x80x94Co alloy maintains a face-centered cubic material structure throughout an operating temperature range of the glow plug so that the resistor spiral is not broken.
From growing awareness of environmental concerns in the recent years, it has become necessary to keep heating the glow plug for a longer time than before, after the starting of the diesel engine, so that exhaust gas from the engine becomes cleaner. It is therefore being demanded that the resistive material of the electric resistor attains not only a quick heating ability and a self-temperature control function but also high durability. However, any conventional resistive material disclosed in e.g., the above patent documents is easily oxidized due to its relatively high iron content. Namely, the conventional resistive material does not have enough oxidation resistant for long-time heating.
In addition, the resistive material with its high cobalt content has come Into use, particularly for the electric resistor that performs the temperature control function, as disclosed in the above patent documents. However, when the resistive material contains a relatively large amount of cobalt, its workability becomes lowered. It is therefore difficult to shape the material into a finer wire, especially by cold-working, to be used as the electric resistor of coil type. This results in failure to meet an increasing demand of downsizing the glow plug.
In view of the foregoing, the present invention has been made to provide a glow plug with an electric resistor, which is free from the above-described drawbacks and can satisfy the following requirements [1] to [3] adequately.
[1] The electric resistor has a high resistance ratio so that the glow plug can attains an excellent quick heating ability.
[2] The electric resistor has high oxidation resistance so that the glow plug has a long life even when used for long-time heating.
[3] The electric resistor can be shaped into a fine wire so that the glow plug can be downsized.
To achieve the above objects, the invention provides a glow plug having an electric resistor that comprises 20 to 60% by weight nickel, less than 5% by weight iron, and the balance being cobalt and unavoidable impurities.